Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session J30: Focus Session: Magnetic Imaging and Characterization |
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Sponsoring Units: GMAG DMP Chair: Yves Idzerda, Montana State University Room: 206B |
Tuesday, March 3, 2015 2:30PM - 2:42PM |
J30.00001: Moment Mapping of bcc Fe$_{\mathrm{1-x}}$Mn$_{\mathrm{x}}$ Alloy Films on MgO(001) Yves Idzerda, Harsh Bhatkar, Elke Arenholz The magnetic moments of $\sim$ 20 nm single crystal films of compositionally graded Fe$_{\mathrm{1-x}}$Mn$_{\mathrm{x}}$ films (0.1 $\le $ x $\le $ 0.2) grown on MgO(001) are determined by spatially resolved moment mapping using X-ray absorption spectroscopy (XAS) and magnetic circular dichroism (MCD). RHEED measurements confirmed that the growth of Fe$_{\mathrm{1-x}}$Mn$_{\mathrm{x}}$ films remained epitaxial and in the bcc phase up to x$=$0.35 but, like Fe growth, is rotated 45 degree with respect to the MgO(001) surface net. This is beyond the bulk bcc stability limit of x$=$0.12. Both magnetometry and XMCD measurements show that the net magnetic moment of these alloy films behave similarly to the bulk behavior, with a gradual moment reduction at low Mn concentrations followed by an abrupt departure from the Slater-Pauling curve and disappearance of the moment at x$=$0.15. By generating a compositional variation around this critical concentration and subsequently using spatially resolved mapping of the X-ray absorption at the Fe and Mn L$_{3}$-edge using linear and circular polarized soft X-rays, the local composition and elemental moments can be simultaneously mapped across the surface of the sample. The Fe moment is found to gradually reduce with increasing Mn content with a very abrupt decline at x$=$0.15. Surprisingly, the Mn moment shows a very small net moment (\textless 0.1 mu$_{\mathrm{B}})$ at all compositions, suggesting a complicated Mn spin structure. [Preview Abstract] |
Tuesday, March 3, 2015 2:42PM - 2:54PM |
J30.00002: Spectroscopically Resolved Imaging of Spin Dynamics in Ferromagnets Using Nitrogen-Vacancy Centers in Diamond Christopher Wolfe, Vidya Bhallamudi, Sergei Manuilov, Hailong Wang, Chunhui Du, Richelle Teeling-Smith, Carola Purser, Andrew Berger, Rohan Adur, Fengyuan Yang, P. Chris Hammel Understanding ferromagnetic dynamics is important for the development of future nanoscale spintronic and magnonic devices. Nitrogen-vacancy centers (NV) in diamond provide us with one of the few tools that has been shown to be capable of both broadband spectroscopy of ferromagnetic resonance$^{1}$ and nanometer scale imaging of magnetic fields$^{2}$. Towards the goal of studying ferromagnetic dynamics at the nanoscale, we report recent results of the local spectroscopy of spin dynamics in ferromagnetic materials using NV centers. We see an especially strong coupling to low frequency excitations such as domains and domain walls, and observe a clear spatial dependence of the excitation of different spin wave modes. \\\\ $^1$C.S. Wolfe, V.P. Bhallamudi, H.L. Wang, C.H. Du, S. Manuilov, R.M. Teeling-Smith, A.J. Berger, R. Adur, F.Y. Yang, and P.C. Hammel, Phys. Rev. B \textbf{89}, 180406(R) (2014) \\ $^2$G. Balasubramanian, I.Y. Chan, R. Kolesov, M. Al-Hmoud, J. Tisler, C. Shin, C. Kim, A. Wojcik, P.R. Hemmer, A. Krueger, T. Hanke, A. Leitenstorfer, R. Bratschitsch, F. Jelezko, and J. Wrachtrup, Nature \textbf{455}, 648 (2008) [Preview Abstract] |
Tuesday, March 3, 2015 2:54PM - 3:06PM |
J30.00003: A nitrogen-vacancy center magnetometer for measuring magnetization dynamics in ferromagnetic nanostructures Jason Liu, Masaki Nagata, R.D. McMichael During the past decade, research into nitrogen-vacancy (NV) centers, a known defect in diamonds, has increased in popularity. This popularity is due to the ability to optically prepare and measure the magnetic state of the spin triplet associated with the NV center [1]. Optically, the ground state electrons can be excited by a 532 nm wavelength laser and the fluorescence results in the emission of red photons. Optically detected magnetic resonance (ODMR) is possible because the $m_{\mathrm{s}} =$ $\pm$ 1 states create weaker fluorescence. Magnetometers with detectivity on the order of 10 nT/Hz$^{1/2}$ have been demonstrated [2]. In this talk, the design and performance of a scanning diamond NV center magnetometer for magnetization dynamics in ferromagnetic samples will be presented. In this instrument, a microwave magnetic field is used to excite precession in magnetic nanostructures and resulting shifts in the stray field are detected by changes in the ODMR of the diamond NV centers. \\[4pt] [1] M. W. Doherty, N. B. Manson, P. Delaney, F. Jelezko, J. Wrachtrup, L. C. L. Hollenberg, Phys. Rep., \textbf{523} (2013)\\[0pt] [2] L. Rondin, J.-P. Tetienne, T. Hingant, J.-F. Roch, P. Maletinsky and V. Jacques, Rep. Prog. Phys. 77 056503 (2014) [Preview Abstract] |
Tuesday, March 3, 2015 3:06PM - 3:42PM |
J30.00004: Imaging Magnetic Vortices Dynamics Using Lorentz Electron Microscopy with GHz Excitations Invited Speaker: Yimei Zhu Magnetic vortices in thin films are naturally formed spiral spin configurations with a core polarization pointing out of the film plane. They typically represent ground states with high structural and thermal stability as well as four different chirality-polarity combinations, offering great promise in the development of spin-based devices. For applications to spin oscillators, non-volatile memory and logic devices, the fundamental understanding and precise control of vortex excitations and dynamic switching behavior are essential. The compact dimensionality and fast spin dynamics set grand challenges for direct imaging technologies. Recently, we have developed a unique method to directly visualize the dynamic magnetic vortex motion using advanced Lorentz electron microscopy combined with GHz electronic excitations. It enables us to map the orbit of a magnetic vortex core in a permalloy square with \textless 5nm resolution and to reveal subtle changes of the gyrotropic motion as the vortex is driven through resonance [1]. Further, in multilayer spin-valve disks, we probed the strongly coupled coaxial vortex motion in the dipolar- and indirect exchange-coupled regimes and unraveled the underlying coherence and modality [2]. Our approach is complementary to X-ray magnetic circular dichroism and is of general interest to the magnetism community as it paves a way to study fundamental spin phenomena with unprecedented resolution and accuracy. Collaborations with S.D. Pollard, J.F. Pulecio, D.A. Arena and K.S. Buchanan are acknowledged. \\[4pt] [1] S. D. Pollard, L. Huang, K. S. Buchanan, D. A. Arena {\&} Y. Zhu, Direct dynamic imaging of non-adiabatic spin torque effects, Nat. Commun. 3, 1028 (2012).\\[0pt] [2] J. F. Pulecio, P. Warnicke, S. D. Pollard, D. A. Arena {\&} Y. Zhu, Coherence and modality of driven interlayer-coupled magnetic vortices, Nat. Commun. 5, 3760 (2014). [Preview Abstract] |
Tuesday, March 3, 2015 3:42PM - 3:54PM |
J30.00005: Direct imaging of interacting vortex orbits and deformations with Lorentz transmission electron microscopy Shawn Pollard, Javier Pulecio, Yimei Zhu Understanding the interactions between confined, interacting magnetic quasiparticles, such as magnetic vortices, is essential towards developing both an understanding of their mutual coupling, as well as limitations for a variety of spintronic devices. However, due to a lack of spatial resolution afforded by traditional techniques, direct observation of the changes of vortex orbits in real space has been lacking. Utilizing high resolution Lorentz TEM, we image the time averaged vortex trajectories in multi-vortex permalloy rectangles and ellipses while applying an oscillating in-plane field tuned to the vortex gyrotropic mode. Using an additional in-plane DC field, we observe a transition of the vortex orbits from circular to heavily distorted as the vortices are driven together, a result of increased interaction strength in laterally coupled vortex pairs. Furthermore, in closely spaced vortex pairs, the strong coupling results in a single resonance frequency. As the vortices are moved apart, pinning effects begin to dominate, and the peak frequency is no longer singular. Micromagnetic simulations are utilized to further elucidate the coupled behavior and obtain time-resolved information of the dynamic process. [Preview Abstract] |
Tuesday, March 3, 2015 3:54PM - 4:06PM |
J30.00006: Imaging Nano- and Micrometer-sized Magnetic Insulator Devices in the Presence of Spin-Torque Aaron Rosenberg, Colin Jermain, Katja Nowack, John Kirtley, Hanjong Paik, Sriharsha Aradhya, Hailong Wang, John Heron, Darrell Schlom, Fengyuan Yang, Dan Ralph, Kathryn Moler Recent results demonstrate that a giant spin-hall effect in Tantalum can produce large spin torques. We intend to employ this large spin torque to manipulate the magnetic moment in electrically insulating ferrimagnetic Lu$_{\mathrm{3}}$Fe$_{\mathrm{5}}$O$_{\mathrm{12}}$ (LuIG) and Y$_{\mathrm{3}}$Fe$_{\mathrm{5}}$O$_{\mathrm{12}}$ (YIG) devices. Using a scanning SQUID microscope, we can study the possibility of performing reversible switching between magnetic states of nano- and micrometer-sized iron garnet devices induced by current pulses applied to a Tantalum layer in contact with the devices by directly imaging the magnetic state of the device before and after a current pulse. Successful manipulation of magnetic insulators by electrical pulses can be a platform for magnetic memory devices and spintronics. [Preview Abstract] |
Tuesday, March 3, 2015 4:06PM - 4:18PM |
J30.00007: Imaging of spin waves in atomically designed nanomagnets Anna Spinelli, Benjamin Bryant, Fernando Delgado, Joaqu\'In Fern\'andez-Rossier, Alexander F. Otte Exploring the transition from individual quantum spins to classical magnetism is crucial for the development of nanoscale magnetic memory storage solutions. Our aim is to search for signs of collective spin behavior in magnetic lattices built on a surface. Using the tip of a low temperature scanning tunneling microscope (STM), we position Fe atoms on a Cu$_2$N/Cu(100) network with atomic precision, to build ferromagnetically coupled spin chains up to 6 atoms that exhibit bistable behavior. Using a combination of inelastic electron tunnelling spectroscopy and spin polarized STM, we are able to probe the spin dynamics during the magnetization reversal of the whole chain, after a local excitation. Our experiments allow us to observe the nodal structure of the standing spin waves confined inside the chain, and, through combination with theoretical calculations, we can understand their role in making the system switch from one metastable magnetic state to the other. [A. Spinelli et al., Nature Materials 13, 782 (2014)] [Preview Abstract] |
Tuesday, March 3, 2015 4:18PM - 4:30PM |
J30.00008: Mapping electronic ordering in chromium in 3D with x-ray microdiffraction Ruqing Xu In the antiferromagnetic state of chromium, electrons form spin-density waves and charge-density waves with wave vector along one of the lattice cubic axes; the spontaneous ordering of the electrons breaks the lattice symmetry and creates domains within a single crystal. We report the first 3-dimentional mapping of charge-density wave domains in bulk polycrystalline chromium samples using differential-aperture x-ray microdiffraction at the Advanced Photon Source. [Preview Abstract] |
Tuesday, March 3, 2015 4:30PM - 4:42PM |
J30.00009: A Nuclear Magnetic Resonance Force Microscope for Micron-scale Liquids Felipe Giraldo, Jeremy W. Paster, Daniel M. Tennant, John T. Markert We have designed and constructed a Nuclear Magnetic Resonance Force Microscopy (NMRFM) probe for the analysis of liquid and soft matter samples. This NRMFM probe uses a magnet-on-cantilever geometry and is equipped with dual $x$-$y$-$z$ piezoelectric motion stages, for micron-step coarse positioning and sub-nanometer fine positioning of both the laser interferometer and the sample with respect to the cantilever, permitting three-dimensional scanning-mode detection of nuclear magnetism. The probe keeps the cantilever detector in high vacuum, maintaining a high $Q$, while the local NMR properties of nearby aqueous samples in glass microtubes are measured. The entire probe head fits in either a 3.5-cm bore magnet or in an electromagnet with a similarly small gap. We plan to demonstrate the ability to scan and distinguish microscale NMR properties using a copper sulfate solution with concentrations in the 2-20 millimolar range, thus providing dynamical imaging of regions with differing longitudinal relaxation times, $T_1$. This concentration range will permit us to compare the conventional saturation-recovery pulse sequence with a more efficient single-pulse detection, possible when $T_1$ is comparable to or less than the duration of the modified cyclic-adiabatic-inversion pulse. [Preview Abstract] |
Tuesday, March 3, 2015 4:42PM - 4:54PM |
J30.00010: Study of ac-plane Magnetic Microstructure of Fe$_{3}$GeTe$_{2}$ Using Magnetic Force Microscopy Neliza Leon Brito, Eric D. Bauer, Filip Ronning, Joe D. Thompson, Roman Movshovich In the quest to develop design principles governing high performance rare earth-free ferromagnets our group has focused on materials where electronic correlations and crystal environment lead to high magnetic anisotropy. The present study concentrates its efforts on one of these materials, the layered itinerant ferromagnet Fe$_{3}$GeTe$_{2}$, which has a high degree of magnetic anisotropy and an easy magnetization direction along the c-axis. Magnetic force microscopy was used to observe the ground state magnetic microstructure of the a-c plane, and its evolution in an external magnetic field along the b-axis. We built a ``surface magnetization loop'' based on the MFM data from -5 Tesla to $+$5 Tesla. We will discuss our results in view of the bulk magnetization data obtained with a superconducting quantum interference device magnetometer. [Preview Abstract] |
Tuesday, March 3, 2015 4:54PM - 5:06PM |
J30.00011: Exploring magnetic excitations in the condensed matter using single electron spins Toeno van der Sar, Francesco Casola, Ronald Walsworth, Amir Yacoby Pushing the frontiers of condensed-matter magnetism requires tools to probe magnetic excitations on the nanometer scale. We have developed a new approach to exploring magnetic excitations in correlated-electron systems [1], using magnetometry based on single electron spins in diamond. We demonstrate the power of this approach by detecting spin-wave excitations in a ferromagnetic microdisc with nanoscale spatial sensitivity over a broad range of frequencies and magnetic fields. In addition, we show how spin-wave resonances can be exploited for on-chip amplification of microwave magnetic fields, allowing strongly increased spin manipulation rates and single-spin magnetometry with enhanced sensitivity. These results can be directly applied to nanoscale magnetic imaging of spin-wave propagation and magnetic vortex/skyrmion dynamics, and open the way towards spin-spin coupling via ferromagnets.\\[4pt] [1] T.~van~der~Sar, F.~Casola, R.~Walsworth, and A.~Yacoby, arXiv:1410.6423 (2014). [Preview Abstract] |
Tuesday, March 3, 2015 5:06PM - 5:18PM |
J30.00012: Nucleation and Control of Magnetic Quasi-particles via Extrinsic and Intrinsic Energies Javier Pulecio, Peter Warnicke, Dario Arena, Mi-Young Im, Shawn Pollard, Peter Fischer, Yimei Zhu Magnetic quasi-particles present an excellent opportunity to study fundamental magnetic properties and dynamics. The fine balance of energies including demagnetization, direct exchange, external perturbations, crystalline anisotropy, indirect exchange, and DMI, allows for the nucleation of a diverse ensemble of spin textures such as vortices, merons, and skyrmions, all of which demonstrate unique behavior. We present our investigations of single vortex symmetry breaking under external perturbations and demonstrate a method to determine the core polarity using Lorentz Transmission Electron Microscopy [1]. We also discuss how to tailor the high-frequency dynamics of coupled coaxial vortices using indirect exchange interactions [2]. We conclude by discussing the nucleation of unconventional chiral spin textures in nano-disc heterostructures using a complementary multi-technique approach, i.e. micromagnetic modeling, FMR, MFM, MTXM, and LTEM. \\[4pt] [1] J.F. Pulecio, S.D. Pollard, P. Warnicke, D.A. Arena, Y. Zhu, Appl. Phys. Lett. 105 (2014) 132403. doi:10.1063/1.4893422.\\[0pt] [2] J.F. Pulecio, P. Warnicke, S.D. Pollard, D.A. Arena, Y. Zhu, Nat. Commun. 5 (2014) 3760. doi:10.1038/ncomms4760. [Preview Abstract] |
Tuesday, March 3, 2015 5:18PM - 5:30PM |
J30.00013: Single spin relaxometry of spin noise from a ferromagnet Francesco Casola, Toeno van der Sar, Ronald Walsworth, Amir Yacoby The introduction of new schemes for the measurement of spatially resolved dynamic magnetic properties of strongly correlated electrons is essential for the study of condensed matter magnetism and the development of novel spintronic devices. Here we show the possibility to detect the magnetic spin noise produced by a thin ($\sim$ 30 nm) layer of a patterned micro-sized ferromagnet (Ni$_{81}$Fe$_{19}$) by optical initialization and read-out of the single spin state of a nearby nitrogen vacancy center (NV) in diamond. For the interpretation of our results, we develop a general framework describing single-spin stray field detection in terms of a filter function sensitive mostly to spin fluctuations with wavevector $\sim 1/d$, where $d$ is the NV-ferromagnet distance. Our results pave the way towards quantitative and non-perturbative detection of spectral properties in nanomagnets, establishing NV center magnetometry as an emergent probe of collective spin dynamics in condensed matter [1].\\[4pt] [1] T.~van~der~Sar, F.~Casola, R.~Walsworth, and A.~Yacoby, arXiv:1410.6423v2 (2014). [Preview Abstract] |
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